The injured soldiers were flown out in helicopters to Tezpur. Three of them were admitted to a hospital there, while the other three critically injured soldiers have been flown to the Army's Command Hospital in Kolkata.
The incident took place in an area bordering China, about 35 km from Tawang-Gamrala area, killing one jawan and seriously injuring six others, Defence spokesperson of Tezpur 4 Corps, Lt Col Sombit Ghosh, told PTI.
According to sources, a technical defect may have caused the mortar shell to fall short of this expected range. Both the gun and the ammunition will be tested for defects. If the ammunition is found defective, the whole batch will be sent back to the supplier.
A court of inquiry has been ordered into the incident and findings are expected to be in within in 10 days. A decision will be taken after the court of inquiry gives its findings and recommendations.
"Such incidents are not completely unusual but the loss of lives is always unfortunate," said a retired artillery officer of the army.
The families of all the soldiers have been informed about the incident.
Tawang is one of the remotest outposts, where the army men serve in extreme conditions. Also called Little Tibet, Tawang is controlled by India but is claimed by China and is one of most contentious issues in the India-China relations.
In January 2016, two Border Security Force men were killed and five others injured as a mortar shell accidentally went off in Rajasthan's Jaisalmer district. The blast took place during a training exercise in a firing range in Kishangarh.